Automobile airbag systems have been developed to protect vehicle occupants in the event of a crash by rapidly inflating a cushion or bag between a vehicle occupant and the interior of the vehicle. The inflated airbag absorbs the vehicle occupant's energy to provide a gradual, controlled deceleration and provides a cushion to distribute body loads and keep the occupant from impacting the hard surfaces of the vehicle interior.
The use of such protective gas-inflated airbags to cushion vehicle occupants in crash situations is now widely known and well documented. The requirements of a gas generant used in an automobile airbag inflator are very demanding. The gas generant must have a burning rate such that the airbag is inflated rapidly (within approximately 30-100 milliseconds) and the burning rate must not vary over long term storage (aging and/or thermal cycling) or as a result of shock and vibration encountered during the life of the vehicle. The burning rate must also be relatively insensitive to changes in humidity and temperature. When pressed into pellets, wafers, cylinders, discs or whatever shape, the hardness and mechanical strength of the gas generant bodies must be adequate to withstand the conditions to which they will be exposed without any fragmentation or change of exposed surface area. Excessive breakage of the generant bodies will lead to system failure where, for example, an undesirable high pressure condition will be created within the inflator, possibly resulting in catastrophic rupture of the inflator housing.
The gas generant must efficiently produce a relatively cool, non-toxic, non-corrosive gas which is easily filtered to remove solid and liquid combustion by-products. This filtering is needed to preclude damage to the inflatable airbag or injury to the occupant of the automobile. These requirements limit the applicability of many otherwise suitable chemical compositions, shapes and configurations from being used in automotive airbag inflators. Gas generants can also be used for fire extinguishing. Recently, a number of companies have begun using the gases produced by solid energetic or pyrotechnique materials for fire extinguishing.
An important parameter relating to gas generants is physical stability of the gas generant pellet. As mentioned above, physical forces, such as vibration, can abrade or crack the gas generant pellets. This damage is unacceptable as the surface area is increased and thus the ballistics (rate of combustion) are altered. Ballistics can also be altered through the absorption of water and thermal cycling. It is known that most non-azide based gas generants, especially 5-aminotetrazole, are hygroscopic and soften upon heating. These changes cause the gas generant pellet to degrade or crumble. This change in surface area can result in catastrophic failure of the inflator housing due to excessive pressure build up in the housing at the time of ignition.
A source of water for degradation of a generant pellet is the gas generant itself. Many non-azide gas generants are prepared by an aqueous mixing process. Water is used to mix the non-azide fuel, oxidizer, and other components of the gas generant composition. The majority of the water is removed during a drying step, however, at least 1% by weight and sometimes as high as 5% by weight water still remains in the generant composition. This drying step is expensive and dangerous. Any method that would allow the gas generant to be prepared without the use of water would be readily accepted by the industry.
In its broadest aspect, the present invention overcomes the previously described problems through the use of guanidines as the fuel and an oxidizer system comprising strontium nitrate and ammonium perchlorate. In addition, the invention, as it relates to the inflator housing, comprises the use of a metal ribbon with a plurality of apertures and a segment of expanded metal that is rolled into a coil and used as a filter to trap combustion products. The following discussion of the background art will assist the artisan in understanding the advancements that the present invention brings to the industry.